An electrical double layer capacitor has active carbon or other polarizable electrodes as the positive and negative electrodes, and an electrolyte solution consisting of a quaternary onium salt of boron tetrafluoride or phosphorus hexafluoride dissolved in an organic solvent such as propylene carbonate. In such an electrical double layer capacitor, the electrostatic capacity is the electrical double layer at the boundary between the electrode surface and the electrolyte solution, with the advantage that since there is no reaction involving ions as in a battery, the charge-discharge characteristics are good, and there is little capacity deterioration attributable to the charge-discharge cycle.
Consequently, electrical double layer capacitors are indispensable for example in storage systems for fuel cell automobiles and hybrid automobiles for example, and especially in regenerative energy storage systems, which collect the energy that is dissipated during breaking. However, double layer capacity provides lower energy densities than batteries, making it inadequate as a power source for electrical automobiles, so there is a need for further improvements in storage capacity density.
This electrical double layer capacitor comprises an electrode formed with an electrical double layer, in other words a polarizable electrode, an electrolyte solution, a separator passing only ions of the electrolyte, and a collecting electrode for collecting and removing charge from the polarizable electrode, and is configured as a cell with the electrolyte solution sealed in a structure consisting of a pair of polarizable electrodes having collecting electrodes on their reverse faces which are arranged facing each other with a separator between the two.
Various proposals have been made for such electrical double layer capacitors with the aim of providing an electrical double layer capacitor having a large storage capacity. In one example, carbon nanotubes were used as the polarizable material of an electrical double layer capacitor (Patent Document 1).
However, the problem with the invention described in Patent Document 1 above is that an electrode with good capacitance characteristics cannot be obtained because a resin component is used for the binder.
To improve on this, one proposal is to attach and bond conductive fibers or conductive tubes to an electrode substrate roughly parallel to the longitudinal direction of the electrode substrate without using a binder, conductive auxiliary material or the like, as shown in Patent Document 2. However, in the invention of this Patent Document 2 conductive fibers or conductive tubes are attached and bonded to an electrode substrate by an electrodeposition method such as electrophoresis, and complex and time-consuming operations are required for attaching and binding to the electrode substrate, such as agitating a solution of dispersed conductive fibers or conductive tubes in an organic solvent with ultrasound.    Patent Document 1: Japanese Patent Application Laid-open No. 2005-136020    Patent Document 2: Japanese Patent Application Laid-open No. 2006-222175